Dye forming compositions utilizing hexaarylbiimidazole compounds in admixture with a leuco dye, as well as other additives, are known. A “dye forming” composition is one that contains at least one relatively colorless compound, for example a leuco dye, that can form color as a result of application of energy to the composition. Many of these compositions are less sensitive to radiation in the longer wavelength range of the ultraviolet spectrum. Dye forming compositions disclosed in Sheets, U.S. Pat. No. 4,622,286 and Dessauer U.S. Pat. No. 4,311,783 contain a leuco dye and a 2,4,5-triphenylimidazolyl compound as defined therein, exhibit more spectral sensitivity in longer wavelength regions of the spectrum. In addition, the dye forming compositions have increased radical reactivity. Such dye forming compositions are particularly useful in proofing papers, printout paper, overlay films, etc.
These dye forming compositions are applied to permeable substrates, e.g., cellulosic substrates, to form an element capable of forming print-out images. A “print-out image” is an instantly accessible, discernible colored pattern resulting from exposure to electromagnetic radiation, for example, ultraviolet radiation. The ideal cellulose containing substrate possesses a number of attributes including, but not limited to, appropriate caliper, basis weight, smoothness, dimensional stability, “hand”, color, opacity, solvent holdout, and chemical stability in the presence of the photosensitive dye forming compositions. Products currently manufactured include proofing papers which are coated on one or both sides with a dye forming composition. In the marketplace, two-side coated papers are more preferred, but one-side coated papers are less expensive and are more suitable to the workflow of many customers. It has been found, however, that the one-side coated papers may have a tendency towards greater instability with time.
Although substrates have been found that have almost all the attributes outlined above, they fall short in that the dye forming compositions when applied to them can demonstrate physical and/or chemical instability, i.e., the photosensitivity or photoresponse changes over time. It is believed to be the interaction of the permeable substrate and the dye forming composition that causes these instabilities.
Accordingly, there is a need for dye forming composition coated elements having improved photosensitive stability over time for forming print-out images.